We present broadband photometry in the optical, near infrared and sub-millimetre, and mid-infrared spectrophotometry of a selection of carbon stars with optically thin envelopes. Most of the observations were carried out simultaneously. Beside the emission features at 11.3 \mu m due to silicon carbide grains in the circumstellar environment, many of our mid-infrared spectra show an emission feature at 8.6 \mu m. All the observed spectral energy distributions exhibit a very large far-infrared flux excess. Both these features are indeed common to many carbon stars surrounded by optically thin envelopes. We modelled the observed spectral energy distributions by means of a full radiative transfer treatment, paying a particular attention to the features quoted above. The peak at 8.6 \mu m is usually ascribed to the presence of hydrogenated amorphous carbon grains. We found also that the feature at 8.6 \mu m might be reproduced by assuming that the stars have a circumstellar environment formed of both carbon and oxygen rich dust grains, although this is in contrast with what one should expect in a carbon-rich environment. The far-infrared flux excess is usually explained by the presence of a cool detached dust shell. Following this hypothesis, our models suggest a time-scale for the modulation of the mass loss rate of the order of some 10^{3} years.